Conventionally, there has been a twin-clutch transmission which is capable of alternately engaging and disengaging two clutches in parallel with shifting operation so that gear speed (gear ratio) can be sequentially changed without interrupting driving force, and in which a first clutch is disposed between the output shaft of an engine and a first input shaft (first clutch output shaft) and a second clutch is disposed between the output shaft of the engine and a second input shaft (second clutch output shaft), a pair of first speed gears and a pair of third speed gears are disposed between the first input shaft and the first and second output shafts, and a pair of second speed gears and a pair of fourth speed gears are disposed between the second input shaft and the first and second output shafts (see Japanese Laid-Open Patent Publication (Kokai) No. H11-051125, for example).
According to the above prior art, after transmission gears connected to the other one of the first and second clutches are changed while driving force from the engine is being transmitted by one of the first and second clutches, the other one of the first and second clutches is engaged while the one of the first and second clutches is being disengaged so that gear speed can be changed without interrupting driving force.
In the conventional twin-clutch transmission constructed as described above, the total length (axial length) can be reduced since the pairs of transmission gears are arranged with the two output shafts. However, only four forward speed gear ratios can be obtained, and hence satisfactory acceleration performance and fuel economy cannot be achieved.
Also, the conventional twin-clutch transmission has no means for mechanically locking the output shafts, and hence an automobile cannot be reliably parked on a sloping road or the like and the safety cannot be ensured.
The present invention has been devised in view of the above conventional problems, and one of its object is to provide a transmission which can obtain forward sixth and seventh speed gear ratios suitable for automobiles so as to improve acceleration performance and fuel economy as compared with the above prior art transmission, and also reduce the total length as much as possible so that the transmission can be mounted even on a front-wheel-drive vehicle of an engine transverse type.
Also, another object of the present invention is to provide a means for mechanically locking output shafts so as to improve the safety in parking on a sloping road or the like.